Metrology devices are known to measure spatial coordinates of an object under test. Examples of metrology devices include laser rangefinders, laser range scanners, photogrammetry cameras, theodolites and electronic autocollimators. For example, a laser rangefinder typically determines the spatial coordinates of an object under test based on a laser beam reflected from points on the object under test. The laser rangefinder generally measures the distance to the object under test by computing a time of arrival of a transmitted pulsed laser beam.
Metrology devices typically operate using a direct line-of-sight to the object under test. In some cases, metrology devices are operated in a cryogenic environment in which the metrology device may be housed in a pressure tight enclosure (PTE). In such an environment, the metrology device may transmit a light beam to the object under test and receive a light beam reflected from the object under test, by using one or more optical windows in the PTE for interrogating the object under test.
There is a need to measure the radius of curvature (RoC) of at least one mirror segment (or PMSA) of a primary mirror (PM) assembled and aligned into a telescope under cryogenic conditions. Measuring the RoC of more than one mirror segment of the PMSA is beneficial in aligning the mirror segments to each other. Additionally, having measured RoC values of all of the mirror segments in the cryogenic environment is useful when diagnosing problems that may be encountered during the alignment process of the mirror segments. If an instrument, external to the center of curvature of the PMSA, is used to measure the radius of curvature, there are limitations in creating reference features that may be viewed from the instrument. This limitation pushes the measurement towards a solution that incorporates the instrument into a region near the center of curvature.
There are several conventional methods of measuring the radius of curvature of a PM. For example, the radius of curvature of a primary mirror may be set mechanically using a spacer rod between a verification hologram and a reference point at the PM vertex with a length equal to the desired radius of curvature. Other locations for the hologram or PM may be used provided that the proper rod length is calculated based on the geometry of the nominal shape of the PM surface and prescription of the hologram.
Fabricating a long spacer rod becomes impractical as the spacing between the PM and the center of curvature null increases. In addition, a spacer rod presents risk to the optical components since the PM is often located below the spacer rod. The spacer rod tip may cause damage to the coating or glass as it contacts the PM surface. Using a spacer rod at cryogenic temperatures requires the use of remotely operated precision positioning hardware, as well as a spacer rod that is calibrated for use at cryogenic temperatures.
As another example, laser finders may be used in conjunction with an interferometer to measure the spacing between the PM and a null assembly. When an interferometer and a null assembly are interferometrically nulled against the PM, a laser finder may be used to measure the 6-DOF (degrees-of-freedom) position of the null and the PM by measuring the coordinates of three or more corner cubes attached to each PMSA. A coordinate measuring machine (CMM) is typically used to measure the relationship between the corner cubes and the boundaries of the PM to which the corner cubes are mounted.
This approach for measuring the radius of curvature requires use of one or more laser finders housed inside a pressure tight enclosure (PTE) operating in a cryogenic environment. Disadvantageously, the PTE needs power and cooling lines. In addition, if the laser finders are mounted on different isolation platforms they become subject to vibration errors. There is also uncertainty associated with the placement of the corner cubes and the CMM measurements of the corner cubes relative to the mirror.
Yet another approach for measuring the radius of curvature of a PM uses a concave spherical mirror of known radius attached to a center-hole fixture that is disposed on the surface of the annular PM. When an interferometer and null assembly are interferometrically aligned to the spherical mirror, the RoC of the PM may be indirectly determined. The position of the spherical mirror is known with respect to three spherical feet supporting the center-hole fixture. Since the center-hole fixture sits at the center of the PM and on the PM surface, any measurement between the null assembly and the sphere may be converted to a distance between the null assembly and the PM.
The center-hole fixture, which is metallic, and the spherical mirror would be exposed to a cryogenic environment. Extensive calibration of the metal fixture and the glass spherical mirror is needed to make this approach viable. Additionally, when a PM is measured while mounted in a telescope, the center hole is typically inaccessible and not visible from the location of the interferometer/null. Use of the center-hole fixture with three spherical feet is only effective when used with a monolithic PM.
As will be explained, the present invention provides a system for measuring distance to an optical surface using a range finder, such as a Leica ADM (absolute distance meter), which transmits a pulsed laser beam to the optical surface.